The purine bases adenine and guanine, which are essential components of DNA, RNA and high energy phosphorylated compounds such as ATP and GTP, are either salvaged and re-utilized in the production of ribo- or deoxyribonucleotides or degraded by the enzyme xanthine oxidase to a relatively insoluble product, uric acid. As a first step in eliminating this intracellularly formed urate from the body, urate must exit cells. Thereafter, in most mammals, a large fraction of the extracellular urate enters peroxisomes of hepatocytes where it is oxidized by the enzyme uricase to a water soluble product, allantoin, which is then excreted by the kidneys. In other vertebrates, notably humans, some non-human primates, birds and reptiles, uricase is not expressed and, therefore, uric acid is the end product of purine metabolism. In all species, uric acid is ultimately cleared from the extracellular compartment via both the kidneys and intestine, with the former being the predominant excretory route.
Although all cells of the body that contain xanthine oxidase have the capacity to generate and accumulate urate intracellularly during the process of purine metabolism, there is no information on the mechanism(s) by which urate is transported out of cells into the extracellular compartment. Since the solubility of urate is quite low, an efficient mechanism must exist to prevent intracellular urate accumulation during periods of normal as well as accelerated nucleic acid turnover. Similarly, despite the important contribution of the intestine in the clearance of extracellular urate, with as much as one-third eliminated via this route in humans, there is minimal information on the mechanism(s) by which urate is transported by intestinal cells. In contrast, the participation of the kidney in disposing of urate has been extensively examined in multiple species. It is now generally accepted that urinary urate excretion occurs by a complex process that includes filtration at the glomerulus, and tubular reabsorption and secretion that take place primarily within the convoluted portion and pars recta of the proximal tubule. Two modalities of transport have been described in renal cortical cell membranes, an electroneutral anion exchanger that transports urate in exchange for a variety of organic and inorganic anions and an electrogenic urate transporter, a uniporter. Neither of these transporters have been identified and characterized at the molecular level.
The alteration of urate hemeostatis may result in elevated plasma levels of uirc acid, a condition known as hyperuricemia. Some hyperuricemia has a genetic basis, See, e.g. Scegmiller (1975) Arthritis Rheum. 18:743. Hyperuricemia may also result from the use of cytotoxic antineoplastic agents, or from diseases characterized by accelerated destruction of cells and increased turnover of nucleic acids, for example myeloproliferative disorders, leukemias, chronic hemolytic anemias, and multiple mycloma. Manifestations of hyperuricemia include gout and nephrotoxicity. Gout results from an inflammatory reaction to crystals of sodium urate deposited in joint tissue. Urate nephropathy results from precipitation of urate crystals in the renal tubules. Also, uric acid stones are common in the urinary tracts of patients with hyperuricemia. Urate transport may also be negatively affected by the use of nucleotide analogs in therapy of diseases such as HIV.
Accordingly, there is a need in the art for the characterization of urate transport mechanisms. The present invention provides nucleic acids encoding a urate transporter.
The present invention provides isolated nucleic acids encoding mammalian urate transporters (UATs). Such nucleic acids include those encoding the rat and human urate transporter. The present invention further provides vectors comprising the nucleic acids, host cells comprising the nucleic acid, and host cells comprising the vectors. Compositions comprising the vectors and a carrier are also provided.
In another embodiment, the present invention provides isolated and substantially purified urate transporters. Proteoliposomes containing substantially purified urate transporters are also provided, as well as compositions comprising the urate transporter and a carrier.
The present invention further provides methods of making a urate transporter, and methods of identifying agents that inhibit or agonize the transporter.